nixpart0: Don't search for libudev using SO major.

The SO major is going to change in the upcoming update of the Hetzner
rescue system, which will cause NixOps to break because it's statically
using the SO major 0 while the new rescue system will have the major
number 1.

I'm still keeping the udevSoMajor attribute to retain backwards-
compatibility with older NixOps versions.

Signed-off-by: aszlig <aszlig@redmoonstudios.org>